X-ray detectors comprising a directly converting semiconductor layer enable individual X-ray quanta or photons to be detected quantitatively and energy-selectively. In the case of this type of X-ray detectors, an incident X-ray photon generates free charge carriers in the form of electron-hole pairs in the semiconductor layer on account of in part multistage physical interaction processes with a semiconductor material. By way of example, semiconductor materials in the form of CdTe, CdZnTe (CZT), CdTeSe, CdZnTeSe, CdMnTe, InP, TIBr2 or HGI2 are suitable for detecting X-ray photons since these materials have a high X-ray absorption in the energy range of medical imaging. As an example, CZT is unique compared with silicon and germanium detectors in that it operates at room temperature and is capable of processing more than one million photons per second per square millimeter. The spectroscopic resolution of CZT outperforms that of most other detectors.
In order to detect the absorption events corresponding to an X-ray photon, electrodes are fitted to two sides of the semiconductor layer and a voltage is applied to the electrodes in order to generate an electric field. For the spatially resolved detection of the absorption events, one electrode is embodied in pixelated fashion and is designated as a read-out electrode. Another electrode arranged opposite to it is usually embodied in planar fashion and is designated as a counter electrode. In an electric field generated between these electrodes (i.e. anode and cathode), liberated charge carriers are accelerated depending on type of charge and polarity at the electrodes, where they induce electrical signals in the form of currents. The currents are converted, e.g., by means of an evaluation unit, into an evaluation signal, the magnitude of which is proportional to the area integral of the current curve and thus proportional to that quantity of charge which is liberated by an incident X-ray photon. The evaluation signal thus generated is subsequently conducted to a pulse discriminator, which, in a threshold-value-based manner, detects the X-ray.
Photon-counting energy-resolved spectral computed tomography (PCS-CT) detectors are expected to be the next big step in medical CT. Since individual photons are to be counted and characterized coarsely as to their energy at photon count rates of 20 Mcps/mm2 or more, such detectors have to be built with direct converting materials like Cd[Zn]Te instead of scintillators which exhibit scintillation (the property of luminescence when excited by ionizing radiation) and which are much slower than CZT. For future garnet based scintillators this may change, however, for such devices also very fast photodiodes are needed, which have to convert the light into electrical signals. Hence, currently direct conversion materials like Cd[Zn]Te remain a favourable option to for PCS-CT.
However, CZT material is prone to polarization, i.e. formation of a space-charge within a pixel volume when irradiated with X-rays, which space-charge weakens the applied electrical field so that electron-hole pairs, which are generated in an interaction of an X-ray photon with the CZT material, no longer drift fast enough towards the anode and cathode. As a consequence, insufficient or even no charge is collected, and the measurement result is simply wrong. This is illustrated in FIG. 4a which shows measured counts over 12000 short measurement periods of 100 μs on 4 different energy channels Ch1 to Ch4 at four different respective thresholds (30 keV, 33 keV, 36 keV, 51 keV). Shortly before 2000 measurement periods the X-ray irradiation starts (a shutter is opened), and at about 7000 measurement periods the X-ray irradiation stops (the shutter is closed). Due to polarization effects in the detecting material volume after the onset of the X-ray irradiation, the number of counts breaks down for later measurement periods. The upcoming space charge weakens the electric field so that fewer electrons reach the anode, which manifests itself in a reduced number of counts. Depending on the actual threshold value this break down is observed at different points in time.